1. Field of the Invention
This invention relates to a charging device for electrically charging a charged object adapted to be electrically charged while moving in a predetermined direction. It also relates to an image forming apparatus having such a charging device.
2. Related Background Art
Conventionally, in image forming apparatus such as copying machines and printers to which an electrophotography process is applied, an image is formed on a recording medium by electrically charging the surface of a revolving photosensitive body by means of a charging device, exposing the surface of the photosensitive body to light to form an electrostatic latent image on the surface thereof, developing the electrostatic latent image to produce a developed visible image by applying a developing agent, transferring the developed image onto a recording medium such as a sheet of recording paper and fixing the transferred image. Available charging devices employ either a contact charging method or a non-contact charging method. With the contact charging method, an electrically conductive member, which is in fact semiconductor, is arranged as charging member on the surface of the photosensitive body that is the charged object so as to contact the photosensitive body and the photosensitive body is electrically charged by applying a voltage to the charging member and causing electric discharges to take place in micro-gaps near the contact areas. With the non-contact charging method, on the other hand, an electrically conductive member, which is in fact semiconductor, is arranged as charging member near the surface of the photosensitive body that is the charged object but separated from the photosensitive body and the photosensitive body is electrically charged by applying a voltage to the charging member and causing corona discharges to take place.
With either charging method, it is necessary to electrically charge the surface of the photosensitive body so as to make it show a uniform electric potential in order to obtain a good image. Particularly in the case of the contact charging method, it is effective to apply a charged bias voltage obtained by combining a DC component and an AC component to the charging member from the viewpoint of preventing the surface of the photosensitive body from being unevenly charged as a result of voltage application. However, when a voltage obtained by superimposing an AC component on a DC component is applied, there arises a problem that the number of times of ion collisions at the surface of the photosensitive body increases to quickly degrade the photosensitive body rather than when a voltage obtained only by using a DC component is applied without superimposing an AC component. There also arises a problem that the volume of the discharge products adhering to the surface of the photosensitive body also increases. The discharge products adhering to the surface of the photosensitive body obstruct the exposure to light of the image to give rise to defective images called deletion (white space on a colored background).
It is known that these problems become remarkable particularly when the inter-peak voltage Vpp of the AC component that is superimposed on the DC component is large. On the other hand, uneven electric discharges occur when the inter-peak voltage Vpp is too small. For this reason, it is preferable that the lowest voltage that provides uniform electric discharges (to be referred to as discharge triggering voltage Vth hereinafter) is selected as inter-peak voltage Vpp for the AC component and superimposing such an AC component on the DC component. However, the discharge triggering voltage Vth can easily be influenced by changes in the environment including those of temperature and humidity, changes in the electric resistance of the charging member that can be caused by dirt and changes in the state of the surface of the photosensitive body that occur as a function of time in service. In other words, it can vary remarkably. Therefore, the inter-peak voltage Vpp needs to be defined with a wide margin. In reality, a value much higher than the discharge triggering voltage Vth is inevitably selected for the inter-peak voltage Vpp to allow excessive electric discharges to occur between the charging member and the surface of the photosensitive body. Thus, the problem of early degradation of the photosensitive body and that of defective images due to discharge products are still remarkable.
In an attempt for suppressing excessive electric discharges as much as possible, there has been proposed a technique of applying a charged bias voltage obtained by superimposing an AC voltage on a DC voltage and controlled for a constant electric current (see Patent Document 1 listed below). However, the proposed technique can only reduce variances in the discharge triggering voltage Vth and does not drastically dissolve the problems. As an improvement to the technique of Patent Document 1, there has been proposed a technique of selecting a lower value for the inter-peak voltage Vpp by observing the electric current of the AC component when applying a charged bias voltage to the charging member and detecting excessive electric discharges from minute fluctuations of the observed electric current of the AC component (see Patent Document 2 listed below). However, it is indispensable for the technique of Patent Document 2 to detect minute fluctuations of the electric current, which is an extremely difficult job, and the problems as pointed out above arise when such fluctuations are missed. There has also been proposed a technique of directly observing the electric potential of the surface of the photosensitive body for the purpose of selecting a lower value for the inter-peak voltage Vpp (Patent Document 3). However, a device for directly observing the electric potential of the surface of a photosensitive body can hardly be made compact and hence such a technique cannot be employed in an image forming apparatus that needs to be down-sized.
[Patent Document 1]
Japanese Patent Application Laid-Open Publication No. 1-267667
[Patent Document 2]
Japanese Patent Application Laid-Open Publication No. 10-232534
[Patent Document 3]
Japanese Patent Application Laid-Open Publication No. 9-185219